When the first platypus skin reached England at the end of the eighteenth century, George Shaw of the British Museum, who described it in 1799, took a pair of scissors to it. He was looking for stitches. A duck’s bill sewn onto the body of some small furred animal was exactly the kind of thing a traveller might sell a naturalist, and he wanted to find the seam before anyone else did. There was no seam. More skins came, none of them had seams either, and the animal had to be admitted to exist. It took most of another century to settle that it also lays eggs. Shaw was right that something had been stitched together. He was looking for the join in the animal, and it was in the list of what a mammal was allowed to be.